1. Field of the Invention
The present invention relates to semiconductor energy detectors using a charge coupled device (CCD).
2. Related Background Art
Charge coupled devices (CCDs) for use as image sensing devices or the like are the ones that transfer a group of analog electrical charge carriers in a single direction in away synchronous with clockpulses, which devices are capable of converting space information into time-sequence signals. Note here that mere transfer of charge carriers upon simultaneous illumination onto a CCD can lead to what is called the xe2x80x9csmearxe2x80x9d due to mixture of charge carriers optically excited or pumped at respective portions with the charges thus transferred, which in turn results in degradation of image signals. To avoid this, it is generally used to operate with time-division into a charge integration period for performing sensing or pickup of an image (energy ray image detection) and a charge transfer period for performing transfer of optically excited carriers. Practically implementable image pickup devices include frame transfer type (FT type), full-frame transfer type (FFT type), and interline transfer type (IT type) and others, by way of example. Of these ones, FFT type CCDs are mainly used for measurement purposes. The FFT type CCDs are adaptable for use in measuring light of low intensities because of the fact that these have no storage area and are capable of enlarging photosensitive areas to thereby permit the optical use-efficiency to stay higher.
In some cases, as semiconductor energy detectors such as CCDs used for wafer and/or photomask (reticle) inspection or else in the technical field of semiconductor manufacturing architectures, those with high sensitivities for ultraviolet rays (e.g., high-pressure mercury arc lamp xe2x80x9cgxe2x80x9d-line with a wavelength of 436 nm, high-pressure mercury arc lamp xe2x80x9cixe2x80x9d-line of 365-nm wavelength, 308-nm XeCl excimer laser, 248 nm KrF excimer laser, 193 nm ArF excimer laser, etc.) are required in view of the fact that inspection is made by use of a light source for photolithography.
One of such image sensors is a CCD of the back illuminated type (e.g. Published Japanese Patent Application No. 6-29506). In the front side type CCD, transfer electrodes covering a photosensitive area are formed for example of polycrystalline silicon electrodes; however, the resulting sensitivity for ultraviolet radiation or else can decrease due to the fact that such polycrystalline silicon, in particular, absorbs incident energy rays of large absorption coefficient such as those rays with a wavelength less than or equal to 400 nm.
In contrast, the back illuminated type CCD is the one that makes use of a substrate with a thickness of approximately 10 to 30 xcexcm for CCD formation and performs an image sensing/pickup operation upon receipt of incident energy rays from the back surface thereof; accordingly, it is possible to detect the light or else without obstruction by transfer electrodes as disposed on the front side, which in turn makes it possible to realize the intended CCD having high sensitivities even with respect to short-wavelength light (e.g. as less as about 200 nm) such as ultraviolet radiation. Such CCD is also effective for illumination of energy rays with large absorption coefficients such as y rays and/or charged particle rays in addition to the ultraviolet radiation. Optionally, it may also be applied as a CCD of the electron bombardment type.
In regard to the above-stated transfer electrodes made of polycrystalline silicon, there is a problem that the electrical resistivity is great as compared to the resistivity of metals. In particular, in cases where high-speed charge transfer is carried out in a vertical shift register of the CCD, the charge transfer speed or rate can be limited by the wiring resistivity of this polycrystalline silicon. In addition, a clock signal due to a transfer voltage being externally applied can often decrease in waveform sharpness in accordance with the large length of a wiring, resulting in distortion of waveform at certain locations, which in turn leads to occurrence of a difference in its rise-up time thus causing the CCD""s transfer efficiency (ratio of charge transferred between potential wells) decreasing accordingly. Regarding this waveform sharpness reduction or xe2x80x9crounding,xe2x80x9d this is determinable by not only the resistivity but also a capacitance in combination therewith; however, a change in capacitance causes the CCD to likewise vary in amount of transferable charge therein so that the above-noted problem cannot be eliminated thereby.
For such the problem, CCDs using lower resistivity-reduced transfer electrodes having an intermediate layer made of a metal or metal silicide, a multilayer structure or a plated structure are set forth, for example, in Published Japanese Patent Application Nos. 63-46763 and 6-77461.
However, in these structures, shapes of electrical wirings using metals or the like are limited to specific shape and width similar to those of the transfer electrodes. At this time, in case picture elements or xe2x80x9cpixelsxe2x80x9d are highly miniaturized in order to increase the image resolution, resultant interconnect wirings can decrease in width: even in the case of using metallic wirings as discussed previously, such wirings still suffer from an increase in resistivity, thereby making it impossible to obtain any sufficient charge transfer speeds. Additionally, in the case of large-area CCD chips also, an increase in wiring length would result in occurrence of similar high-resistivity problems.
The present invention has been made in view of the aforementioned problems and its object is to provide a semiconductor energy detector capable of transferring charge carriers at high speeds with high efficiency.
To attain the foregoing object, a semiconductor energy detector in accordance with the present invention is the semiconductor energy detector having on a front side of a CCD a group of transfer electrodes. Auxiliary wirings are directly connected to some of the transfer electrodes, and the other transfer electrodes are conencted via the additional wirings and corresponding transfer electrodes. While the auxiliary wirings and additional wirings are for causing the impedance due to the transfer electrodes to decrease to thereby enable achievement of high-speed/high-efficiency charge transfer, employing this wiring structure makes it possible to widen these wirings while at the same time improving the charge transfer at high speeds with high efficiency.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.